1. Field of the Invention
This invention relates to novel poly(amide acid) and poly(imide acid) compositions and mixtures thereof useful as release agents for pressure-sensitive adhesive tapes.
2. Description of the Prior Art
Various polyamide and polyimide resins are described in the prior art, including a large class of nylon type resins prepared from aliphatic dicarboxylic acids and aliphatic diamines. There is also a large class of so-called linear polyamides and polyimides which are formed by the reaction of polycarboxylic acids and anhydrides and polyprimary amines. Compounds of this latter type are usually prepared by the reaction of tetracarboxylic acid dianhydrides and primary polyamides and products of this type are disclosed, for example in U.S. Pat. Nos. 3,094,511, 3,154,524, 3,179,614, 3,502,762 and in the textbook entitled "New Linear Polymers" by Lee et al, published by McGraw Hill (1968). These linear or "wholly aromatic" polymers have been found useful in a wide variety of areas including use as a backing member for pressure-sensitive adhesive tapes.
With respect to this specific prior art, U.S. Pat. No. 3,154,524 relates to polyamide resins of this general class produced by the reaction of polyamines and aromatic polycarboxylic acids obtained from certain solvent extracts. These products are said to be particularly useful in protective coatings, structural plastics and the like. In U.S. Pat. No. 3,502,762 polyimide films are provided by the reaction of carboxylic acid dianhydrides and unsubstituted organic polyamines which products are said to be useful, for example as a backing member for normally tacky pressure-sensitive adhesive tapes.
Release coating agents for pressure-sensitive adhesive tapes are, in general, used to prevent, or at least reduce, the transfer of the adhesive mass during unwind from a roll thereof, to the backing member, and to control the unwind tension. A release coat provides suitable release properties only if it is less adherent to the adhesive mass than is the backing member itself. However, the release coat must have sufficient adhesion to the backing member to prevent "pick-off" thereof by the adhesive mass. And, it must have a suitable amount of adhesion to the adhesive mass otherwise it will unwind too readily when in roll form. Thus, it is seen that a release coating to be suitable must involve a balancing of various desired characteristics. It has been found, however, that with some release agents, although initially good release characteristics are provided; after extended contact with an adhesive mass, particularly at elevated temperatures, undesirable side effects are noted. Thus, the adhesion between the release coat and adhesive mass may be so great as to make unwinding very difficult, if not impossible. Various other disadvantages associated with the use of release agents now known to the prior art are also known.
The present invention provides a pressure-sensitive adhesive tape comprising a novel release coating which is neither disclosed nor suggested in any of the above-identified references or any other references of which Applicant is aware. This novel class of release agents, as disclosed hereinafter, provides a pressure-sensitive adhesive tape which unwinds easily after storage even in a warm environment, and the adhesive exhibits substantially unimpaired tack and adhesion.